1. Field of Invention
The invention relates to communications networks and, more particularly, to a method and apparatus for allocating beacon signals in a communications network.
2. Description of Related Art
In wireless communications networks, beacon signals, which are sometimes also referred to as pilot signals, are broadcast over the airwaves to enable mobile devices to search for cell sites that are handoff candidates. Generally, one beacon signal is required for each sector of each cell site and each beacon signal of a given cell site uses a different frequency. FIG. 1 is a diagram of a plurality of cell sites, each of which is divided into three sectors with each sector of a given cell site using a different frequency. The frequencies that are allocated for use as beacon signals for the network are frequencies f1 through f12. For cell site 1, which is shown in block diagram form in FIG. 2, the beacon signal for sector a uses frequency f1 and is broadcast over the sector by directional antenna 11. The beacon signal for sector p uses frequency f2 and is broadcast over the sector by directional antenna 12. The beacon signal for sector y uses frequency f3 and is broadcast over the sector by directional antenna 13. The next adjacent cell site 2 uses frequency f4 for sector a, frequency f5 for sector p and frequency f6 for sector y. The next adjacent cell site 3 uses frequency f7 for sector a, frequency f5 for sector 13 and frequency f9 for sector y. The next adjacent cell site 4 uses frequency f10 for sector a, frequency f11 for sector [3 and frequency f12 for sector y.
After frequencies f1 through f12 have been used for the beacon signals for cell sites 1 through 4, those frequencies are then reused in non-adjacent cells. For example, cell site 1, which uses frequencies f1 through f3 for the beacon signals is separated from cell site 5, which also uses frequencies f1 through f3 for the beacons signals, by cell site 4, which uses frequencies f10 through f12 for the beacons signals. As the number of frequencies used for the beacon signals increases, the possibility that co-channel interference will occur decreases. However, increasing the number of frequencies that are reserved for the beacon signals decreases the number of frequencies that can be used for caller traffic. Therefore, with the current beacon signal allocation scheme, a tradeoff exists between spectral efficiency and co-channel interference.
It can be seen from FIG. 1 that each cell site uses three different frequencies for the beacon signals and that the beacon signal frequencies are reused after twelve frequencies have been used. After frequencies f1 through f12 have been used for the beacon signals for cell sites 1 through 4, those frequencies are then reused in non-adjacent cells. For example, cell site 1, which uses frequencies f1 through f3 for the beacon signals is separated from cell site 5, which also uses frequencies f1 through f3 for the beacons signals, by cell site 4, which uses frequencies f10 through f12 for the beacons signals. Therefore, in the beacon signal frequency allocation scheme shown in FIG. 1, twelve frequencies are allocated to four cell sites. This is commonly referred to as a 4/12 beacon signal allocation configuration, where X=4 corresponds to the number of cell sites that use a given frequency before the frequency is reused and Y=12 corresponds to the total number of frequencies used. Other beacon signal frequency allocation schemes that are common are 5/15 and 7/21.
FIG. 2 is a functional block diagram of cell site 1 shown in FIG. 1. As shown in FIG. 2, the user frequencies start at frequency f13 because frequencies f1 through f12 are reserved for the beacon signals. For exemplary purposes, FIG. 2 depicts only three frequencies (f13 through f15) being used for caller traffic and the same user frequencies being used for caller traffic for all of the sectors. The beacon signals are continuously broadcast by the base stations of the cell sites at constant power so that the mobile devices are able to easily detect the beacon signals. In order to avoid interference, the frequencies that are used for the beacon signals are not used for caller traffic. Typically, frequency hopping is used so that the same frequency is not being transmitted at the same time in two sectors of the same cell site. For example, if user frequencies f13 and f14 are being transmitted over sector α by antenna 11 in time slots t1 and t2, respectively, user frequencies f14 and f13 may be transmitted over sector β by antenna 12 in time slots t1 and t2, respectively, but not in time slots t2 and t1, respectively.
A signal combining and distribution unit 10 receives the beacon signals generated by beacon logic 14 and caller traffic signals generated by caller traffic logic 15. The signal combining and distribution unit 10 combines the beacon and caller traffic signals for broadcasting by the respective antennas 11, 12 and 13 over the respective sectors, α, β and γ. The beacon signals are normally broadcast in time slot t1 of a Broadcast Common Control Channel (BCCH) frame that is made up of eight time slots of equal duration, t1 through t8. For this reason, the beacon logic 14 is referred to herein as BCCH logic.
FIG. 3 depicts a BCCH frame 21. The first time slot, which corresponds to t1, is reserved for transmission of BCCH information, which includes the beacon signal. Time slots t2 through t8 can be used to transmit information other than BCCH information, including caller traffic. However, even when there is no caller traffic, all eight time slots t1 through t8 are transmitted so that mobile devices can detect the beacon signal. In order to ensure that mobile devices can easily detect the beacon signal, the BCCH frame 21 is continuously broadcast.
One of the disadvantages of allocating beacon signals in the manner described above with reference to FIGS. 1-3 is that a relatively large number of frequencies must be reserved for use as beacon signals, which reduces the spectral efficiency of the network. For example, with respect to the configuration shown in FIG. 1, twelve frequencies are reserved for the beacon signals. If these frequencies were not reserved for use as beacon signals, they could be used for other purposes, such as caller traffic, which would allow each cell site to service more calls. Therefore, allocating a large number of frequencies for use as the beacon signals reduces the number of calls that can be handled by each cell site, thereby reducing network coverage and increasing overall network costs.
Another disadvantage of the current beacon signal allocation scheme is that transmission of the BCCH frame even when there is no caller traffic is inefficient in terms of power consumption. In addition, because the first time slot t1 is reserved for transmission of the BCCH frame, the first time slot cannot be used for other purposes, such as for caller traffic. For this additional reason, the current scheme of beacon signal frequency allocation is not spectrally efficient.
Another disadvantage of the current beacon signal allocation scheme is that it requires time consuming and tedious frequency planning to ensure that all BCCH channels in the same cell site have sufficient frequency separation to avoid adjacent channel interference impact (i.e., interference between the signals in adjacent sectors of the same cell site). In addition, sufficient distance separation is necessary to reduce co-channel interference. As shown in FIG. 1, cell sites that use the same frequencies for the beacon channels are not located adjacent one another, but are separated by some distance. This is because co-channel interference will result if cell sites that use the same frequencies are located adjacent one another. The frequencies used for the beacon signals and the locations of the cell sites are chosen so that co-channel interference is reduced to acceptable levels.
Accordingly, a need exists for a method and apparatus for beacon signal allocation that provide a spectrally efficient way of performing beacon signal allocation that is also efficient in terms of power consumption.